RV Repair: Gray water tank, rv supply, 5th wheel

Question
I have a 1995 25’ Jayco Eagle 5th wheel that I recently purchased used. I found that when you only put a few gallons of water into the gray water tank the gauge shows full. The gauge display shows 3 lights to show how full the tank is after only a few gallons all 3 lights come on. Could this be fault of the display, or tank? What all is involved with making this repair? If the tank has to be removed what has to be done to do this? Would there be connection at the top of the tank and if so how do you get to them? Thanks for any help you can give me. Over filled it 2 times now and water backs up into my shower.
Thank you.  

Answer
There could be 2 problems.  1, most common is the probes on the inside of the tank could be dirty.  You can get probe cleaner at any RV supply, add the recommended amount and some water, add a bag of ice to help with the "scrubbing" action, and drive the trailer around, dump and check the panel.

The other thing to check, there is a resister "pack" on the wiring at the tank, you can mark each location on the tank, and remove the wires.  the lowest wire will be the ground wire.  touch the ground wire to each other wire one at a time and have someone check the panel, you should see each stage light up with each wire touched.  If it does not, then the resister pack is defective.

Garry